Uncertainty in the hydrologic and soil erosion predictions of the WEPP watershed model due to errors in model parameter estimation is identified through a sensitivity analysis based on the Monte Carlo method. Hillslope component model sensitivities to model inputs for rangeland conditions are presented. Model sensitivities provide guidance in the collection of input data where the model is intended to simulate soil erosion. The results show that hydrologic and erosion predictions are very sensitive to attributes that define a storm event (amount, duration, and time to peak and intensity) and to the saturated hydraulic conductivity parameter. Sensitivity to critical shear stress in soil erosion predictions indicates that interrill flow is the dominant factor of sediment transport under consolidated, nontill managed soils.